1. Field of the Invention
This invention relates to a method of fabricating lightweight, composite materials, as well as to a method of encapsulating electrical components to provide increased resistance to electrical stress. More particularly with regard to the former, this invention relates to a method of preparing epoxy resin based filler reinforced composite members exhibiting improved performance characteristics such as reduced weight and thermal expansion coefficients.
2. Description of the Prior Art
The successful utilization of filler reinforced composites in aerospace applications, such as antenna fabrication, imposes several important requirements on the filler reinforced composite. A feature of primary importance is that the composite have strength and dimensional stability characteristics (e.g. a low linear coefficient of thermal expansion) calculated to withstand the rigors of environmental temperature cycling.
In addition, for complex structures such as antennae, structures molded from epoxy resin based composites are preferred over metal structures since the composite can be molded directly, thereby avoiding costly machining operations. Glass and graphite fiber reinforced epoxy based composites are finding growing use in aerospace applications because of their high strength-to-weight ratio. These composite materials, because of their relatively low coefficient of thermal expansion (.alpha.), find wide application in structural components such as antenna used in space. Such materials are described, for example, by H. S. Katz and J. V. Milewski, in the book entitled "Handbook of Fillers and Reinforcements for Plastics," Chapter 19, Hollow Spherical Fillers, Van Nostrand Reinhold Company, New York, 1978, pages 326 to 330. In a space environment where one face of the structural member is subjected to constant sunlight while the opposite face of the member is in darkness, the face exposed to sunlight is heated considerably more than the opposed face. Such non-uniform heating causes uneven expansions of the structural members making up the antenna with the resultant distortion of the antenna from its desired shape. Thus, aerospace use requires that some critical antenna structural components exhibit a dimensional stability over a ten year lifetime in the operating range of 54.degree. to 115.degree. F. (12.degree. to 46.degree. C.) and have densities equal to or less than 0.9 grams/cm.sup.3. When the structural components are electroplated with a metal layer to provide, for example, utility as a electromagnetic interference (EMI) shielding or a conductive path in an antenna waveguide structure, the metal coating layer must also withstand thermal cycling in the extremes of the space environment without loss of adhesion.
Furthermore, in another area of current interest, it is recognized that high voltage power supplies and pulse forming networks for aerospace use must meet high standards of performance and reliability for long periods under extreme environmental conditions. To assure trouble-free operation of components within the assembly, the components, such as magnetic coils, capacitors, diode arrays, transformers, stator generators, and resistor networks, are commonly encapsulated with synthetic resin materials to provide electrical insulation to the components. Such encapsulants for electrical and electronic components are described, for example, in the book by Katz et al, previously referenced, at page 327. The conventional encapsulation processes used are batch processes, i.e., processes which are not in continuous production, but rather are carried out on a limited number of items at one time.
Because of their excellent adhesion, good mechanical, humidity and chemical properties, epoxy resins are used extensively both as encapsulants for electronic components and in combination with glass and graphite fibers and microballoons in the manufacture of the previously discussed high performance reinforced composite structures, such as antenna. The encapsulated article or filled epoxy resin composite is molded using conventional molding techniques, such as compression molding or transfer molding, in which the required materials including the resin are loaded into a mold, and curing of the resin is effected under increased pressure, with the load being applied directly to the mold by the action of a contained media, namely a gas for pneumatic pressure or a liquid for hydraulic pressure. In compression molding, the pressure is applied to a platen which then presses on an opening in the mold which communicates with the mold cavity. In transfer molding, the pressure is applied to a transfer ram which then presses on the mold contents through the opening in the mold. Problems encountered in the molding of the epoxy systems and especially filled epoxy resin systems have limited the use of these resins as encapsulants, as well as for the fabrication of structural composites useful in a space environment. In molding filler reinforced composites, the problem is aggravated due to the fact that high filler loadings, e.g. 40 to 60 percent by weight, are required in the epoxy resin based composite structures to achieve the required low .alpha. value and corresponding high dimensional stability. Thus, when using transfer molding or batch encapsulation techniques, the use of high filler contents in the epoxy system impedes adequate resin flow into the mold because of the high viscosity of such a resin system. This, in turn, results in varying filler orientations and distributions within the geometrical areas in the complex structures being molded, with a resultant loss in .alpha. and ultimate dimensional stability, as well as erratic adhesion of metal layers plated on the surfaces of the structure. This problem is further aggravated when hollow particulates such as glass or graphite microballoons are used as filler materials since the high viscosity filled epoxy resin may have unpredictable densities due to microballoon fractures and voids in the filler/resin mixture. Further, high viscosity resins are unable to penetrate and impregnate the filler to provide homogenous systems.
In addition, with regard to encapsulants for electrical components, state-of-the-art heat curable epoxy resin systems used as encapsulating resins have the disadvantage that the viscosity of the resin at working temperatures, e.g. 100.degree. C., (75.degree. F.), are quite high, e.g., 500 centipoise (cps), and such resins are not intruded completely into crevices present in the electrical component. Thus, the insulation is often incomplete and defective.
It is the primary object of the present invention to provide a method for molding filled thermosetting resins such as polyimides, bismaleimides, and particularly epoxy resins, to provide resin based filler reinforced composite structural components which have a low coefficient of thermal expansion rendering the composite relatively insensitive to environmental temperature cycling while possessing desirable performance properties of low density, high strength, and amenability to the plating of adherent metal films and coatings.
It is a further object of the present invention to provide a method for encapsulating electrical components or devices to provide components or devices having improved reliability and improved electrical properties.
Another object of the present invention is to provide methods of the type described above which can be performed in cyclic and continuous fashion.